Probably ~15TB through file-level syncing tools (rsync or similar; I forget exactly what I used), just copying up my internal RAID array to an external HDD. I’ve done this a few times, either for backup purposes or to prepare to reformat my array. I originally used ZFS on the array, but converted it to something with built-in kernel support a while back because it got troublesome when switching distros. Might switch it to bcachefs at some point.
With dd specifically, maybe 1TB? I’ve used it to temporarily back up my boot drive on occasion, on the assumption that restoring my entire system that way would be simpler in case whatever I was planning blew up in my face. Fortunately never needed to restore it that way.